The requirements for increasingly high areal recording density impose increasingly greater demands on thin film magnetic recording media in terms of remanent coercivity (Hr), magnetic remanance (Mr), coercivity squareness (S*), medium noise, i.e., signal-to-noise ratio (SNR), and narrow track recording performance. It is extremely difficult to produce a magnetic recording medium satisfying such demanding requirements.
The linear recording density can be increased by increasing the coercivity of the magnetic recording medium. However, this objective can only be accomplished by decreasing the medium noise, as by maintaining very fine magnetically non-coupled grains. Medium noise is a dominant factor restricting increased recording density of high density magnetic hard disk drives. Medium noise in thin films is attributed primarily to inhomogeneous grain size and intergranular exchange coupling. Accordingly, in order to increase linear density, medium noise must be minimized by suitable microstructure control.
A conventional longitudinal recording disk medium is depicted in FIG. 1 and comprises a substrate 10, typically Aluminum (Al) or an (Al)-alloy, such as an Al-magnesium (AlMg.sub.-- alloy, plated with a layer of amorphous nickel-phosphorus (NiP). Alternative substrates include glass, ceramic and glass-ceramic materials, silicon, plastic, as well as graphite. There are typically sequentially sputter deposited on each side of substrate 10 and adhesion enhancement layer 11, 11', e.g., chromium (Cr) or a Cr alloy, a seedlayer 12, 12', such as NiP, an underlayer 13, 13' such as Cr or a Cr alloy, a magnetic layer 14, 14', such as cobalt (Co)-based alloy, and a protective overcoat 15, 15', such as a carbon-containing overcoat. Typically, although not shown for illustrative convenience, a lubricant topcoat is applied on the protective overcoat 15, 15'.
It is recognized that the magnetic properties, such as Hr, Mr, S* and SNR, which are critical to the performance of a magnetic alloy film, depend primarily upon the microstructure of the magnetic layer which, in turn, is influenced by the underlying layers, such as the underlayer. It is recognized that underlayers having a fine grain structure are highly desirable, particular for growing fine grains of hexagonal close packed (HCP) Co alloys deposited thereon.
It has been reported that nickel-aluminum (NiAl) films exhibit a grain size which is smaller than similarly deposited Cr films which are the underlayer of choice in conventional magnetic recording media. Li-Lien Lee et al., "NiAl Underlayers For CoCrTa Magnetic Thin Films", IEEE Transactions on Magnetics, Vol. 30, No. 6, pp. 3951-3953, 1994. Accordingly, NiAl thin films are potential candidates as underlayers for magnetic recording media for high density longitudinal magnetic recording. However, it was found that the coercivity of a magnetic recording medium comprising an NiAl underlayer is too low for high density recording, e.g. about 2,000 Oersteds (Oe).
Lee et al. subsequently reported that the coercivity of a magnetic recording medium comprising a NiAl underlayer can be significantly enhanced by depositing a plurality of underlayers containing alternative NiAl and Cr layers rather than a single NiAl underlayer. Li-Lien Lee et al., "Effects of Cr Intermediate Layers on CoCrPt Thin Film Media on NiAl Underlayers," Vol. 31, No. 6, November 1995, pp. 2728-2730. It was found, however, that such a magnetic recording medium is characterized by an underlayer structure exhibiting a (110)-dominant crystallographic orientation which does not induce the preferred (1120)-dominant crystallographic orientation in the subsequently deposited Co alloy magnetic layer and is believed to contribute to increased media noise.
Li-Lien Lee et al. were able to obtain an underlayer exhibiting a (200)-dominant crystallographic orientation by initially depositing a Cr sub-underlayer directly on the non-magnetic substrate at an undesirably high temperature of about 260.degree. C. using radio frequency (RF) sputtering. However, deposition of a Cr sub-underlayer at such an elevated temperature undesirably results in large grains, which is inconsistent with the reason for employing NiAl as an underlayer. On the other hand, it is very difficult to obtain a Cr (200)-dominant crystallographic orientation, even at elevated temperature such as 260.degree. C., on glass, ceramic and glass ceramic substrates using direct current (DC) magnetron sputtering, which is widely employed in the magnetic recording media industry.
Li-Lien Lee et al. recognized the undesirability of resorting to high deposition temperatures to obtain a (200)-dominant crystallographic orientation in the underlayer structure. It was subsequently reported that an underlayer structure exhibiting a (200)-dominant crystallographic orientation was obtained by depositing a magnesium oxide (MgO) seedlayer using radio frequency (RF) sputtering. Li-Lien Lee et al., "Seed layer induced (002) crystallographic texture in NiAl underlayers," J. Appl. Phys. 79 (8), 15 April 1996, pp. 4902-4904; and David E. Laughlin et al., "The Control and Characterization of the Crystallographic Texture of the Longitudinal Thin Film Recording Media," IEEE Transactions on Magnetics, Vol. 32, No. 5, September 1996, pp. 3632-3637. Such a magnetic recording medium, however is not commercially viable from an economic standpoint, because sputtering systems in place throughout the industry making magnetic recording media are based upon direct current (DC) sputtering. Accordingly, RF sputtering an MgO seedlayer is not economically viable. The use of an NiAl underlayer is also disclosed by C.A. Ross et al., "The Role Of An NiAl Underlayer In Longitudinal Thin Film Media" and J. Appl. Phys. 81(a), P.4369, 1996.
Various efforts have been made to optimize the magnetic properties of a magnetic recording medium by achieving a desirable crystallographic structure in a magnetic film employed to store information. These efforts involve the use of different materials for the seedlayer, underlayer or buffer layer, as well as varying sputtering parameters, including the substrate temperature, sputtering power density, substrate bias, film thickness, sputtering gas environment and sputtering pressure. In order to achieve a strong in-plane magnetic anisotropy with high Hr and high recording signal, it is necessary to form the easy magnetic axis of the magnetic layer so that it is substantially aligned in the film plane.
As the demand for higher areal recording density increases, the thickness of the magnetic film employed in the magnetic recording medium decreases. However, there is a superparamagnetic limit where the grain size of the magnetic layer becomes less thermally stable with a reduction in grain size. Consequently, a small thermal agitation will deteriorate the stored magnetic information. Moreover, even before the superparamagnetic limit is reached, as the film thickness is reduced, additional factors negatively impact magnetic coupling, such as the smaller grain size and the non uniformity of the films. These factors all reduce in-plane magnetic anisotropy and, consequently, reduce Hr within a certain Mrt range. Currently, for most of the materials, the Mrt range at which a dramatic decrease in Hr is observed is about 0.5 memu/cm.sup.2. In fact, it is typically found that at for most, using current magnetic recording medium manufacturing process, the Hr is reduced by up to about 50% of its maximum value at an Mrt of about 0.4 memu/cm.sup.2.
There exists a need for magnetic recording media having high in-plane anisotropy at a low film thickness. There exists a particular need for magnetic recording media suitable for high areal recording density exhibiting a high Hr at a Mrt less than about 0.5 memu/cm.sup.2.